This invention relates generally to multi frequency vertical deflection signal generator circuits and specifically to vertical signal generator circuits for producing a synchronized sawtooth (ramp) output voltage over a range of input synchronizing signal frequencies.
The prior art includes a number of circuit arrangements using a capacitor that is charged from a constant current source to form the ramp voltage and means for changing the current from the current source in response to changes in the input sync condition (frequency). The ramp generator is often used with upper and lower voltage level sensing comparators that alternately set and reset a flip/flop which, when activated, discharges the ramp capacitor. The arrangement comprises an oscillator that can be triggered by injecting a sync signal, either to the ramp voltage, or directly to the flip/flop, for initiating discharge of the ramp capacitor. This is referred to as an injection locked oscillator system.
Changing the current supplied to the ramp capacitor in an injection locked system results in a change in the free running (no sync) frequency. Changing the ramp current when sync is applied doesn't result in a frequency change, but rather changes the amplitude of the resultant ramp, provided the current adjustment is not so high as to cause the free running frequency to exceed the injected sync frequency.
The prior art discloses many open loop systems that attempt to maintain a constant, or at least a predictable, vertical height under multi frequency operations. Some systems use a microprocessor to control the ramp current, while others use discrete logic to switch between different levels of ramp current. The selection of various levels of ramp current is usually dependent on the frequency and polarity of horizontal and vertical sync. Such systems are subject to error due to variations in capacitors and current sources and changes over time and in temperature and errors in measured sync frequency. Such systems are also subject to errors when used with uncommon or unrecognized display signal formats.